PROJECT SUMMARY/ABSTRACT Metastatic breast cancer (mBC) is the chief cause of mortality among breast cancer (BC) patients. The dismal outcomes of systemic therapies for this disease are due in part to our incomplete understanding of critical interactions between the mBC cells and their microenvironment, in particular of the role of physical forces in disease progression and treatment resistance. The local microenvironment is known to mediate disease progression and treatment resistance differentially in primary versus metastatic BC. In this proposed project, I will explore how the mechanical microenvironment of metastases affects resistance to immunotherapy for mBC. Our lab has previously shown that solid stress ? a newly discovered physical abnormality in tumors, defined as the mechanical pressure accumulated within the solid components of the tumor ? is elevated in primary BCs and causes pronounced vascular compression (PNAS 2012, Nat Commun 2013). This vascular compression leads to decreased blood perfusion and increased hypoxia, both of which could promote BC immunosuppression (PNAS 2011 & 2012). We have discovered that the accumulation of solid stress in primary BCs is due to desmoplasia, characterized by high levels of cancer-associated fibroblasts and extracellular matrix components (PNAS 2012). We have also found that primary BC desmoplasia can be reduced using high doses of angiotensin II receptor 1 blockers (ARBs), but at the cost of adverse effects (hypotension) (PNAS 2011). What remains unknown is whether solid stress is elevated in metastases, at what stage it begins to accumulate, and which components or processes mediate its genesis. Also unclear is whether reduction of solid stress results in reprogramming the immune microenvironment, and eventually enhancing immunotherapy in mBC. Here I propose to first quantify solid stress in mBC using novel high-resolution measurement techniques and mathematical modeling. I will then characterize the changes in stromal components and the immune microenvironment in response to solid stress alterations to identify the consequences of solid stress (Aim 1). In Aims 2, based on promising preliminary data, I will utilize newly developed ARB-based therapeutics that selectively become active in the mBC microenvironment to alleviate solid stress. In doing so, I will create therapies that can target solid stress in mBC while avoiding systemic side effects. I will test whether these agents can reduce solid stress, reprogram the immune microenvironment, and enhance the outcomes of immune checkpoint inhibitors in mBC models. The proposed work will lead to new paradigms for the study of mBC and will improve immunotherapy for this intractable disease.